Link adaptation is an important Radio Resource Management (RRM) function in wireless communication systems for reliable communication. The purpose of link adaptation is to determine the appropriate modulation and coding scheme (MCS) to maximize user data throughput or data rate. In a typical system that utilizes Hybrid Automatic Repeat Request (HARQ), the task of link adaptation is to determine the MCS that will provide the highest data transmission rate for which the targeted operating point, e.g., block error rate (BLER) after certain number of HARQ transmissions, can be achieved. To perform link adaptation, information about the communication link quality is required. This may be obtained either from measurements at the transmitter or reports of Channel State Information (CSI) from receiver to transmitter. To achieve high throughput, link adaptation may be performed in every transmission time interval (TTI).
A downlink (DL) link quality can be determined from reported CSI from user equipment (UE). A methodology for evaluating link quality is not defined by communication standards, and, therefore, can vary from vendor to vendor. Some UEs may report optimistic CSI while other types of UEs may report pessimistic CSI. The CSI reported by the UE is typically mapped to a channel quality measure, e.g., signal to interference plus noise ratio (SINR), by the transmitter. To achieve a target BLER, a control loop, called an outer-loop, may be utilized that makes additional adjustments to the channel quality measure obtained from the reported CSI based on the transmission successes and failures.
For example, SINR is increased by an upward step when a transmission for a transport block (TB) is successful while it is decreased by a downward step when a transmission for a TB fails. A HARQ transmission for a TB is considered successful from an outer-loop perspective if a HARQ feedback of positive acknowledgement (ACK) is received before exceeding a target number of HARQ transmissions. A HARQ transmission for a TB is considered failed if the HARQ feedback is still a negative acknowledgment (NACK) when the target number of HARQ transmission is reached. If no positive acknowledgement (ACK) is received before the target number of HARQ transmission and no HARQ feedback signal is detected when the target number of HARQ transmission is reached, the HARQ transmission for a TB is neither a success nor a failure. The upward adjustment steps are determined based on the target BLER. For example, one downward step is set to nine upward steps with a target BLER of 10%.
In an ideal case, the estimated SINR is perfect. In other words, the actual BLER is exactly the target BLER when the highest MCS supported by the radio link is selected. In such a case, the SINR adjustment by outer-loop converges to zero since the downward adjustment completely cancels the upward adjustment. Thus, the downward adjustment and upward adjustment correspond to the target (and actual) BLER.
In real world deployments, the estimated SINR can be either too high (optimistic) or too low (pessimistic). When the estimated SINR is optimistic, if, for example, the target BLER is 10%, then the actual BLER is higher than 10%, and the overall SINR adjustment by the outer-loop becomes negative, correcting the estimated SINR to make it less optimistic. Similarly, when the estimated SINR is pessimistic, assuming again that the target BLER is 10%, the actual BLER is lower than 10% and the overall SINR adjustment by outer-loop becomes positive, correcting the estimated SINR to make it less pessimistic.
A higher BLER for larger Transport Block Sizes (TBSs), which tend to use higher modulations and coding rates, is a phenomenon that exists in practically all digital radio communication systems. Depending on the specific radio environment and the ability of the system to adapt to a varying channel quality, the magnitude will vary.
Larger transport block sizes that tend to use higher modulation, e.g., 64QAM, are chosen for higher radio qualities (higher CQI values). Due to the varying radio channel, the probability of experiencing a worse radio quality than expected depends on the reported radio quality. The higher the quality that is measured and reported, the higher the probability that the quality will degrade and be lower than when the block is actually transmitted over the radio interface, which is, in other words, an over-estimation of channel quality.
A same contribution is made for the lower part of the radio quality distribution. Smaller block sizes that use the lower modulation, e.g., QPSK, are chosen when the quality is low, and the probability that the channel will be improved at the moment of transmission increases.
Generally, if a UE reports a higher CQI that corresponds to 64QAM, it tends to report the optimistic radio quality. Consequently, the 64QAM would have higher HARQ retransmission rate than 16QAM and QPSK. On the other hand, if a UE reports a lower CQI that corresponds to QPSK, it tends to report the pessimistic radio quality. As a result, the HARQ retransmission rate for QPSK would be lower than that of 16QAM and 64QAM.
Existing solutions use channel quality, e.g., gain to interference plus noise ratio (GINR), estimation and link adaptation that consists of receiving, by the Evolved Node B (eNodeB), a CQI report from the UE. The CQI is mapped to SINR using a table derived from receiver link models. The GINR is then derived from the SINR by subtracting the power spectrum density of the reference signal. A smoothing filter is applied to GINR to allow for slow fading of the adaptation. HARQ ACK/NACK feedback is used to adjust GINR to counteract systematic errors between UE measurement and eNodeB link adaptation.
Thus, the derived GINR estimate is used to calculate link qualities for all modulation modes, e.g., QPSK, 16QAM, and 64QAM. A drawback of this method is that the link quality calculation for the higher throughput modulation, e.g., 64QAM, tends to be overestimated, and the link quality calculated for the lower throughput modulation, e.g., QPSK, tends to be underestimated. The overestimated or underestimated link quality is used by the link adaptation function to determine the largest allowable number of symbols, SBs, and the corresponding MCS that should be used for efficient utilization of resources. Thus, a more aggressive MCS, i.e., having a higher data throughput rate, may be selected for a larger TBS with higher modulation, and a more conservative MCS, i.e., having a lower data throughput rate, may be selected for a smaller TBS with lower modulation. A consequence of such an arrangement is that the actual BLER for a high throughput modulation may be higher than the target BLER, and the actual BLER for a low throughput modulation may be lower than the target BLER, which would degrade the overall cell throughput. A solution to more accurately measure channel quality to improve cell throughput is needed.